This adds support for authenticating with Fedora's OpenID Connect (OIDC) provider.

Existing users will be matched by email address, they should be able to use the new authentication method transparently.

This requires getting a `client_id` and a `client_secret` from Fedora Infra, see `INSTALL.md`.

Signed-off-by: Aurélien Bompard <aurelien@bompard.org>